In U.S. Pat. No. 5,282,204, assigned to Racotek, Inc. of Minneapolis, Minn., a time domain data overlay over trunked radio infrastructures is disclosed. Although this time domain overlay has been found to be excellent in many respects, there are certain deficiencies.
One of the deficiencies of the time domain overlay is that the effective data transmission rates are limited due to the channelized scheme (25-30 Khz channel bandwidth) primarily designed for voice. Using the time domain overlay the data stream is transmitted over a single channel, and the baud rate is limited by the bandwidth of the channel. Typically, using the time domain system, the data transmission rate is 4800 baud. Under certain conditions however, it is highly desirable to transmit larger amounts of data much faster than 4800 baud.
Another deficiency of the time domain data overlay is that it is tightly coupled to the voice infrastructure resulting in high data latency and access time. As additional channels in a cellular radio system are being used, the latency increases exponentially. Since the load/latency characteristics of the voice and data structure affect each other, the result is a requirement for complex load balancing schemes. For example, in the Racotek time domain data overlay system it is recognized that voice should be given priority and that data may not have to be transmitted immediately. If data is to be transmitted, the channels are searched and if there is an available channel, the data is transmitted on that channel. However, if there is no available channel the system backs off and there is a programmed delay before it attempts retransmission. The back off time increases during subsequent attempts to transmit data and, with severe congestion, the back off time may run into minutes. Complex algorithms are required by the Racotek system to balance the channel seeking and back off times to prevent the system from being overloaded, to prevent the system from being driven into instability and to attempt to only marginally affect the existing customer base.
Another deficiency of the time domain overlay is that temporarily unused channels are not able to be pooled to increase the data transmission rates. The effective data rates are limited due to the fact that transmission is only in one channel which is primarily designed for voice transmission.
We have discovered a spectral overlay that serves to alleviate the deficiencies concomitant with a time domain data overlay. The spectral overlay of our invention solves a tremendous need for sending data quickly, inexpensively, and using present infrastructure.
It is, therefore, an object of the present invention to provide a data communication system that is operable to transmit data at very high effective data rates.
Another object of the present invention is to provide a data communication system that alleviates the data delays concomitant with the time domain data overlay systems.
A further object of the present invention is to provide a data communication system in which data may be transmitted at the same time that voice is being transmitted, without overloading the system.
A still further object of the present invention is to provide a data communication system in which temporarily unused channels may be pooled to increase the data transmission rates.
Other objects and advantages of the present invention will become apparent as the description proceeds.